Magnetic Properties of Materials

Materials respond to magnetic fields in different ways depending on the behaviour of the electrons in their atoms. Each electron acts as a tiny magnetic moment because of its spin and orbital motion. How these moments are arranged and whether they align with an applied field determines the magnetic character of a substance. The three classic categories are diamagnetism, paramagnetism and ferromagnetism, with several related forms such as antiferromagnetism and ferrimagnetism.

Diamagnetic materials have no permanent magnetic moment; an applied field induces a weak moment that opposes the field, so they are very slightly repelled. Paramagnetic materials contain atoms with permanent moments that are randomly oriented by thermal motion, but they align weakly with an applied field, producing a small attraction that disappears when the field is removed. Ferromagnetic materials, such as iron, cobalt and nickel, are special because their atomic moments interact strongly and align with one another over regions called magnetic domains. This cooperative alignment can produce strong magnetisation that persists after the field is removed, which is why these materials form permanent magnets and the cores of transformers and motors.

The behaviour of ferromagnets is described by the hysteresis loop, a plot of magnetisation against applied field. As the field is cycled, the magnetisation lags behind, so the material retains some magnetisation (remanence) at zero field and needs a reverse field (coercivity) to demagnetise. Soft magnetic materials have narrow loops and are easy to magnetise and demagnetise, suiting them to transformer cores, while hard materials have wide loops and make good permanent magnets. Heating a ferromagnet above its Curie temperature destroys the ordered alignment, and the material becomes paramagnetic until it cools again.

Frequently asked questions

What makes iron magnetic but copper not?

Iron is ferromagnetic: its atomic magnetic moments align cooperatively within domains, producing strong magnetisation. Copper lacks this alignment and is only very weakly diamagnetic.

What is a magnetic domain?

A region within a ferromagnetic material where the atomic magnetic moments all point the same way. Applying a field grows favourably aligned domains, magnetising the material overall.

What happens at the Curie temperature?

Above the Curie temperature, thermal energy overcomes the alignment of moments, so a ferromagnetic material loses its spontaneous magnetisation and behaves paramagnetically.